Re: [mpls] [sfc] An MPLS Forwarding plane for SFC

"Joel M. Halpern" <> Fri, 01 September 2017 20:25 UTC

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Cc: 'Service Function Chaining IETF list' <>
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From: "Joel M. Halpern" <>
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Date: Fri, 01 Sep 2017 16:25:33 -0400
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Subject: Re: [mpls] [sfc] An MPLS Forwarding plane for SFC
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Reading this draft, what you have proposed is a specific transport 
mechanism, using MPLS.  By removing the NSH header, you remove the 
transport agnostic properties that the Working Group was specifically 
chartered to achieve.
By recasting the metadata into a label sequence, you make any metaata 
processing significantly harder, and make applications dependent upon 
the MPLS transport, rather than being able to rely on the NSH format. 
If this pattern were followed for other transports, we would require SFF 
and SF which understood how to parse and process all of the different 
transport encodings of the path, and SF would have to understand all the 
different transport encodings of the metadata.

Why is this beneficial?

If what you want to do is carry NSH, with an MPLS label stack that 
represents the whole sequence of places to visit, we would still have to 
assume that SF preserved the MPLS stack, but their processing, assuming 
they could find the carried NSH header under the MPLS stack, would at 
least be independent of the transport.


On 9/1/17 4:15 PM, Adrian Farrel wrote:
> Hi,
> We've been working up some ideas for using an MPLS forwarding plane (switching
> or SR) for SFC.
> We have constrained ourselves to the architecture developed by the SFC working
> group, and have used the NSH as a functional model.
> MPLS is somewhat limited compared to the NSH encapsulation, so there is a
> trade-off between using a new encapsulation with full function and a good set of
> function using an existing forwarding plane.
> At the moment this is an early version of our work, but we thought you'd like to
> see our thought processes.
> (FWIW draft-ietf-bess-nsh-bgp-control-plane is applicable to NSH or MPLS
> encapsulations and includes mechanisms to select between the two.)
> Cheers,
> Adrian
>> -----Original Message-----
>> From: I-D-Announce [] On Behalf Of
>> Sent: 01 September 2017 21:00
>> To:
>> Subject: I-D Action: draft-farrel-mpls-sfc-00.txt
>> A New Internet-Draft is available from the on-line Internet-Drafts
> directories.
>>          Title           : An MPLS-Based Forwarding Plane for Service Function
> Chaining
>>          Authors         : Adrian Farrel
>>                            Stewart Bryant
>>                            John Drake
>> 	Filename        : draft-farrel-mpls-sfc-00.txt
>> 	Pages           : 23
>> 	Date            : 2017-09-01
>> Abstract:
>>     Service Function Chaining (SFC) is the process of directing packets
>>     through a network so that they can be acted on by an ordered set of
>>     abstract service functions before being delivered to the intended
>>     destination.  An architecture for SFC is defined in RFC7665.
>>     The Network Service Header (NSH) can be inserted into packets to
>>     steer them along a specific path to realize a Service Function Chain.
>>     Multiprotocol Label Switching (MPLS) is a widely deployed forwarding
>>     technology that uses labels to identify the forwarding actions to be
>>     taken at each hop through a network.  Segment Routing is a mechanism
>>     that provides a source routing paradigm for steering packets in an
>>     MPLS network.
>>     This document describes how Service Function Chaining can be achieved
>>     in an MPLS network by means of a logical representation of the NSH in
>>     an MPLS label stack.
>> The IETF datatracker status page for this draft is:
>> There are also htmlized versions available at:
>> Please note that it may take a couple of minutes from the time of submission
>> until the htmlized version and diff are available at
>> Internet-Drafts are also available by anonymous FTP at:
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